Known surface light sources for use in a back light system for a liquid crystal display device or the like may include those of side edge type, for example, as disclosed in Japanese Patent Application Laid-Open No. Hei 7-159622.
FIG. 1A is a schematic cross sectional view illustrating a surface light source device 200 of such a side edge type.
As shown, the surface light source device 200 can include: a light guiding plate 210 having a wedge-shaped cross section, and a light incident surface 210a with a large width, a tapered surface 210b, and a light exiting surface 210c; a light source 220 disposed along the light incident surface 210a; a reflection sheet 230 disposed on the tapered surface 210b; and a prism sheet 240 having a prism portion 240a, the prism sheet 240 facing downward and disposed over the light exiting surface 210c. In this surface light source device 200, the light beams emitted from the light source 220 can be incident on the light guiding plate 210 through the light incident surface 210a. The light beams in the light guiding plate 210 can be uniformly distributed while reflected by the reflection sheet 230 on the tapered surface 210b, and then exit from the light exiting surface 210c. The exiting light beams from the light guiding plate 210 can be condensed by the prism portion 240a of the prism sheet 240 to be projected toward a liquid crystal display part (liquid crystal display) 300.
The light beams exiting the prism sheet 240 may be directed in various directions including the direction perpendicular to the surface of the liquid crystal display part 300. Namely, the effective light beams with which the liquid crystal display part 300 is illuminated are those incident perpendicularly on the liquid crystal display part 300 meaning that only the light beams perpendicularly exiting from the light exiting surface of the prism sheet 240 are effective for illuminating the liquid crystal display part 300. Accordingly, it is required or at least helpful that the light exiting angle β at which the light beams exit from the light exiting surface 210c of the light guiding plate 210 be limited within a predetermined range so that the exiting light beams from the prism sheet 240 can be directed substantially perpendicularly to the light exiting surface of the prism sheet 240 by the prism portion 240a of the prism sheet 240. Furthermore, the taper angle γ formed between the tapered surface 210b and the light exiting surface 210c of the light guiding plate 210 should be adjusted to be a predetermined angle so that the light beams that pass through the light guiding plate 210 can be reflected to the light exiting surface 210 and exit therethrough by the light exiting angle β, the light exiting angle β being within the above predetermined range.
Recently, surface light source devices with higher front luminance have been demanded because of the development of higher definition liquid crystal displays or the application to head-up displays.
The above-mentioned conventional surface light source device 200 however has had problems when utilizing a high-power light source 220 for achieving higher front luminance. Since such a high-power light source 220 is thicker than before, the taper angle γ of the light guiding plate 210 must be made thicker, thereby increasing the light beams exiting by light exiting angles β out of the above predetermined range. This can hinder in increasing the higher front luminance.
Furthermore, conventional light source devices 200 typically utilize a side-view type light emitting diode (LED) as a light source 220. However, many high-power light sources may be top-view type LEDs having light distribution characteristics that are nearer the complete diffusion than the side view-type LEDs. Accordingly, if the top-view type LED is utilized in order to increase the output of the light source 220, the light beams propagating through the light guiding plate 210 may be likely to rise at an acute angle due to the near-complete diffusion light distribution characteristics. This means the light exiting angle β may become smaller, resulting in an increase in the percentage of the light beams exiting at a light exiting angle β out of the predetermined range. Accordingly, it is difficult to increase the front luminance of such a surface light source device.